The present invention relates to a corona discharge processing apparatus, and more particularly to a corona discharge processing apparatus for improving surface properties of synthetic resin products.
Corona discharge processing is employed for improving the surface properties such as adhesion and coating performance of synthetic resin formings and synthetic resin films, by making use of the corona discharge generated between electrodes when a high voltage is applied.
A structure of a corona discharge processing apparatus is disclosed in the Japanese Official Patent Gazette No. 60-46133. In this apparatus, on a synthetic resin forming supported by a movable base electrode, a counter-electrode is disposed at the opposite side of the base electrode. The forming is placed between the counter-electrode and the base electrode. In this state, when a high voltage is applied between the counter-electrode and the base electrode, corona discharge is generated. By the generated corona discharge, corona discharge processing is applied for the surface of the forming.
In order to enhance the corona discharge effect, it is required that the interval between the base electrode and the counter-electrode be as short as possible. Therefore, the counter-electrode which moves relatively to the forming is made of a flexible material such as a chain and a metal brush, and is designed to move while deforming along the uneven shape of the forming. The base electrode is formed in the same shape as the back side of the forming. As a result, the base electrode and the counter-electrode can just fit to the both sides of the forming, and the interval between the base electrode and the counter-electrode is set to minimum.
In such prior art, however, it is necessary to fabricate base electrodes of different shapes according to the shape of the forming. Therefore, if the shape of the forming is complicated, it takes much time and cost for fabricating the base electrode. Moreover, if the back shape of the forming and the shape of the base electrode do not fit exactly, a gap is produced between the back face of the forming and the base electrode, and tight contact of the forming with the base electrode is not achieved. Thereby, an electric discharge occurs in the gap between the back surface of the forming and the base electrode, and the discharge energy on the processing surface is lowered, and corona discharge processing of the forming may not be done sufficiently, or the processing may be insufficient.
Yet, as thickness of the forming increases, even if the base electrode and the forming are in tight contact, the distance between the electrodes is extended and corona discharge processing may not be done sufficiently. That is, depending on the thickness of the forming, the processing effect varies and also, there is a certain limit to the thickness of the forming that can be processed.
When the shape of the forming is simple like a sheet, fabrication of the base electrode is relatively easy, but if the thickness of the forming is not uniform due to manufacturing error, the processing effect also fluctuates. If thickness of the sheet is more than tens of millimeters, sufficient processing effect will not be attained with an ordinary applied voltage of 20 to 30 kV.